Current transformer for a compensating current sensor

ABSTRACT

The invention relates to a current transformer comprising a primary conductor, a secondary winding and a soft-magnetic core, whereby the primary conductor and secondary winding are electrically insulated from one another and are magnetically intercoupled via the core. In addition, a current to be detected is injected into the primary conductor, and a compensating current is injected into the secondary winding. According to the invention, an additional conductor is provided which is electrically connected in parallel to the primary winding, and which is situated in the area of the core at which a flux rise occurs, whereby the flux rise is at least partially compensated for by the flux elicited by the current flowing in the additional conductor.

[0001] The invention relates to a current transformer for a compensatingcurrent sensor having at least one primary conductor, a secondarywinding, and a soft magnetic core, whereby the secondary winding doesnot completely wind around the core, whereby the primary conductor andsecondary winding are electrically insulated from one another and aremagnetically intercoupled via the core, and whereby the primaryconductor is provided for injecting a current to be detected and thesecondary winding is provided for injecting a compensating current.

[0002] Current sensors are supposed to be small and reasonably pricedbut, at the same time, have a wide measuring range. The use of currentsensors according to the compensation principle (compensating currentsensors) is preferred here. Such a compensating current sensor is knownfrom European Patent 0 294 590, for example. In this patent, a currenttransformer, in which the magnetic field, generated in its soft magneticcore by the . . . by the current to be detected . . . is compensated bya magnetic field generated by a compensating current injected into thesecondary winding. The known compensating current sensor exhibits aring-shaped core, through which the primary conductor leading to thecurrent to be detected is pulled and is covered around the entirecircumference by the secondary winding. Through the latter, an evendistribution of the magnetic flow over the entire circumference of thecore is achieved, as a result of which a wide measuring range isadvantageously achieved.

[0003] However, a complete wrapping of the core for many applications istoo costly and/or difficult to construct. The problem here, however, isthat in current sensors having only a partially covered core, the flowis injected only in a certain area of the core, in which the magneticfield probe is normally arranged. In this area, the magnetic flow iscompensated to zero by the compensating coil. Because the flow of thecompensating coil that only partially encloses the core does notcompletely close the core, but partially closes as stray flux in freespace, the flow in the core is no longer completely compensated outsidethe compensating coil. When the current intensity increases, thematerial can therefore go into saturation outside the compensating coil.The linear measuring range of the current transformer, and consequently,of the entire compensating current sensor, is thus limited by thissaturation effect. However, a wider measuring range is normallydesirable even for these applications.

[0004] It is known that an enhancement of the measuring range may beachieved by dividing the primary conductor, in which a portion of thecurrent is led through the current sensor and a portion is conductedpast the sensor. The enhancement of the measuring range correspondsexactly to the distribution ratio of the current to be measured. Thismeans that with a current distribution of 1:4, a measuring rangeenhancement of 5 is achieved.

[0005] For example, taking the distributed primary conductor around thecore is known from DE-OS 2 303 906. In the process, an AC converter isrealized, equipped with a squirrel cage winding. This attenuates theflow in the core by the quotients from the impedance of this squirrelcage winding and by the ohmic resistance of the squirrel cage winding. Areceiving coil supplies a voltage proportional to the change over timeof the magnet flux, and consequently, the energizing current. On the onehand, this is an arrangement to be operated only with alternatingcurrent, and on the other hand, the output voltage is greatly dependenton the ohmic resistance of the short-circuit ring, and consequently, onthe temperature. Nevertheless, no satisfactory enhancement of themeasuring range is achieved even in this manner.

[0006] The task of the present invention is to specify a currenttransformer for a compensating current sensor, which has a widemeasuring range despite low cost.

[0007] The task is achieved by a current transformer in accordance withPatent claim 1. The embodiments and developments of the inventive ideaare the subject matter of sub-claims.

[0008] The advantage of the present invention is the ability to usesmaller cores and/or an unsymmetrical lapping since saturation effectsand non-homogeneous magnetic fluxes are extensively compensated for.

[0009] In particular, in a current transformer of the type mentioned atthe start, at least one additional conductor is provided, which iselectrically connected in parallel to the primary winding and isarranged in the area of the core where a flux rise occurs, in such a waythat the flux rise is compensated for at least partially.

[0010] The present invention is based on the fact that a currentdistribution, and consequently, a measuring range enhancement, isinitially effected in the conventional sense via the additionalconductor, but that however, the special arrangement of the additionalconductor achieves a measuring range enhancement beyond this. Theadditional conductor led externally, past the core, is now placed suchthat the magnetic flow injected through this portion of the primaryconductor (additional conductor) compensates for the magnetic flux inthe outer areas of the core. This means that the magnetic flow in thecore originating from this portion of the primary conductor (additionalconductor) replaces the stray flux of the compensating coil, which islacking for the compensation. It is in this manner that measuring rangeenhancements that go well beyond the values achievable throughconventional current distribution are achieved.

[0011] In particular, the additional conductor is arranged beside thecore in such a way that only certain areas of the core are detected byits magnetic flow.

[0012] For example, if several increases in the current occur in varioussites of the core, at least one other additional conductor may beprovided, which is electrically connected in parallel [to] the primaryconductor and the additional conductor, and which is arranged beside thecore in such a way that only certain areas of the core arecorrespondingly detected by its magnetic flow.

[0013] Primary conductor and additional conductor(s) preferably conductcurrents of different strengths, with the sum of the currents givingrise to the current to be detected. In this manner, a maximum oflinearity, and consequently, a maximum measuring range, can be achievedfor every geometry (core and windings) through an optimal currentdistribution.

[0014] When primary conductor and additional conductor(s) are not coiledup, i.e., number of turns is less than one, so-called straight-throughtransformers can be realized, for example, in which the primaryconductor leading to the current to be detected is led through by aring-like core.

[0015] In a preferred embodiment, primary conductor and additionalconductor(s) can be formed by an essentially rectilinear conductor inthe area of the core or an essentially U-shaped conductor in the area ofthe core. The U-shaped formation of primary conductor and additionalconductor(s) while interleaving the individual conductors further widensthe measuring range because the U-shape, in particular, causes a slightsaturation of the core.

[0016] If primary conductor and additional conductor(s) are formed by asingle, slotted conductor, there are advantageously no contactresistances changing with the time or the temperature at the otherwisenecessary contact sites of primary conductor and additional conductoramong themselves, as a result of which the measuring accuracy isimproved.

[0017] A return conductor, which is arranged beside the core on the sideof the core opposite the other additional conductor(s), and which istraversed in the opposite direction of the primary conductor andadditional conductors by the current to be detected, can moreoverlikewise increase the measuring accuracy by compensating for negativeflow rises.

[0018] Primary conductor and additional conductor(s), secondary winding,and core, may in the end at least partially, individually or together,be encompassed by a shielding plate. This relieves the core and reducesthe influence of the noise fields that would unfavorably affect themeasuring range.

[0019] The present invention is explained in greater detail in thefollowing, using the embodiments illustrated in the figures of thedrawing.

[0020] Shown are:

[0021]FIG. 1 a first, general embodiment of a current transformeraccording to the present invention,

[0022]FIG. 2 a second embodiment of a current transformer according tothe present invention, with a conductor conducting an opposing current,

[0023]FIG. 3 a third embodiment of a current transformer according tothe present invention, with two U-shaped conductors that areinterleaved,

[0024]FIG. 4 a fourth embodiment of a current transformer according tothe present invention, which, as opposed to the embodiment shown in FIG.3, is enhanced by a further secondary coil,

[0025]FIG. 5 a fifth embodiment of a current transformer according tothe present invention, with a shielding plate,

[0026]FIG. 6 a sixth fifth embodiment of a current transformer accordingto the present invention, with a slotted, one-piece conductor,

[0027]FIG. 7 the format of a preferred, one-piece, slotted conductor indetail,

[0028]FIG. 8 a seventh embodiment of a current transformer according tothe present invention, with two conductors arranged beside the core,

[0029]FIG. 9 the course of the line of electric flux of a currenttransformer according to the present invention, with two primaryconductors, in comparison with a conventional current transformer, and

[0030]FIG. 10 the course of the line of electric flux of a currenttransformer according to the present invention, with shielding plate, incomparison with a conventional current transformer.

[0031] In the embodiment shown in FIG. 1 of a current transformeraccording to the present invention, an additional conductor is provided,aside from a primary conductor and a secondary winding 2, with theprimary conductor and additional conductor each being designed asmicrostrip 1 or 3. Instead of microstrips, any other form of conductorcould be used in the same manner. Secondary winding 2 has a plurality ofwindings and is arranged around this via an air gap (not shown inFIG. 1) of soft magnetic core 4. Core 4 has an essentially rectangularcross-section and is designed in a ring shape, originating from the airgap, in such a way that the base is approximately rectangular. Insteadof a rectangular base and a rectangle-shaped cross-section, however, anyother base and cross-sectional area facilitating a circular shape ofcore 4, such as for example, an oval, round area, etc., could be used.

[0032] In the process, microstrip 1 (primary conductor) penetrates thebase of core 4 in an essentially straight line, in such a way that it issurrounded over the full circumference by its air gap, as seen from core4. Secondary winding 2 surrounds only a part of core 4 (in theembodiment, less than one fourth of the circumference). In accordancewith the present invention, a second microstrip 3 (additionalconductor), which is electrically connected in parallel to the firstmicrostrip 1 (primary conductor), is outside of core 4, but in itsimmediate vicinity, and with respect to the first microstrip 1 (primaryconductor), arranged opposite the secondary winding. Microstrip 3serving as additional conductor generates a magnetic flux in theprocess, which compensates for the uneven flow distribution in core 4,caused by secondary winding 2 only partially enclosing core 4, and forexample, microstrip 1 (primary conductor) arranged unsymmetricallywithin the core.

[0033] The embodiment according to FIG. 2 follows from the embodimentshown in FIG. 1, in that a return microstrip 5 is provided in addition,which is arranged opposite microstrip 3, near the secondary winding 2,and which conducts a current that flows in the opposite direction ofmicrostrips 1 and 3 and is equal to the sum of the currents throughmicrostrips 1 and 3. On account of the position opposite microstrip 3and opposite the current direction, there is on the whole astrengthening of the effect created by microstrip 3 (additionalconductor).

[0034] In the embodiment according to FIG. 3, the microstrips areU-shaped, as opposed to the embodiment shown in FIG. 1. The U-shaperesults in a winding with 0.75 number of turns. Here, the twomicrostrips 1 and 3 continue to run parallel, resulting in twointerleaved “U's”. The distance between the corresponding sides of theU-shaped microstrips 1 and 3 with respect to one another can also bereduced in view of the distance in the area of core 4, by which theinfluence of noise fields caused by the current supply can be reduced.The U-shape on the whole offers a wider measuring range.

[0035] In the arrangement shown in FIG. 4, as opposed to FIG. 3, anadditional secondary coil 2′ is also provided, which is electricallyconnected, for example in series to secondary coil 2. The secondary coil2′ is opposite secondary coil 2 here, or arranged between microstrips 1and 3, enclosing the core 4. This measure also allows the measuringrange to be linearized, and consequently, enlarged.

[0036] The embodiment according to FIG. 5 follows from the embodimentshown in FIG. 1 in that a U-shaped shielding plate 6 surrounds core 4 aswell as microstrips 1 and 3. Shielding plate 6 originates here from thetwo sides of core 4 that are found at the side of secondary winding 2and runs parallel thereto and, including the two microstrips 1 and 3,also parallel to the section of core 4 opposite secondary coil 2.Through shielding plate 6, the influence of noise fields, which couldreduce the measuring range, is decreased and the measuring range isenhanced, moreover, since the core is also relieved.

[0037] In the embodiment according to FIG. 6, microstrip 7 is provided,originating from core 4 and secondary coil 2 of the previousembodiments, the microstrip being slotted in the area of core 4 andforming two partial microstrips resulting in microstrip 1 and 3. Thepartial microstrips are turned by 90 degrees to the non-slotted area ofmicrostrip 7. Because of the one-piece design of microstrip 7, the twomicrostrips 1 and 3 have no points of contact, thereby avoidinginterference-prone points of contact.

[0038] A preferred embodiment of a one-piece implementation ofmicrostrips 1 and 3 as a one-piece, slotted microstrip 8 is shown inFIG. 7. Shown in detail here in FIG. 7a is the structure before and inFIG. 7b the structure of microstrip 8 after performing a bendingprocess. According to FIG. 7a, microstrip 8 has a rectangular basicshape, where lengthwise, or parallel to the longer edges, there arethree slots 9, 10, 11 at approximately half the width, of which a 9 isarranged lengthwise in the middle and two 10, 11 are arranged on theside edges. Microstrip 8 is symmetrically structured with respect to asymmetry line running in its middle at a right angle to the longitudinaldirection as well as a symmetry line running along slots 9, 10, 11.Originating from each of the two side edges containing slots 10 and 11,microstrip 8 is bent at approximately the height of the ends of slots 10and 11 facing the center, in such a way that there is on the whole anindentation opposite the mentioned side edges, with a furtherindentation being provided in the area of slot 9. Finally, near thecorners are holes 12, 13, 14, 15, which are symmetrically arranged atleast with respect to the symmetry line running along slots 9, 10, 11.

[0039] Originating from this basic form, in accordance with FIG. 7b,microstrip 8 is bent at the symmetry line running along slots 9, 10, 11,in such a way that two each of holes 12, 14 or 13, 15 come to be on topof the other. This results in a shape that, originating from the sideedges containing slots 10 and 11, . . . lie close together initially inthe area of holes 12, 13, 14, 15, in order to stand out against oneanother in the first indentation. The distance between the two parts ofmicrostrip 8 is greatest in the area of the second indentation (aroundslot 9). This part serves to accommodate the core.

[0040] In the embodiment shown in FIG. 8, originating from theembodiment shown in FIG. 1, the other microstrip 3 is divided into twopartial microstrips 16 and 17 running parallel to one another, which arepositioned in such a way that they offer an optimal compensation of fluxrises.

[0041]FIG. 9 shows how a flux rise is caused in a current transformer ofthe conventional type (FIG. 9a), for example through unsymmetricalarrangement of a (primary) conductor 18 within a ring-shaped core 19with air gap 20. A secondary coil 21 is coiled up around the core 19 viathe air gap 20, the secondary coil being shown in the drawing in profilefor better overview. At the section of the core 19 (short-circuit core)across secondary coil 21, a flux rise occurs due to a nonpoint-symmetrical arrangement of (primary) conductor 18, the flux risecoming about on account of magnetic flux of (primary) conductor 18.

[0042] If according to the present invention (FIG. 9b), a “dividedconductor”, i.e., the (primary) conductor 18 and an additional conductor22 electrically connected in parallel, are now led through on both sidesof the section of core 19 (short-circuit core) exhibiting the rise, thelines of electric flux cancel each other out again and the resultingmagnetic field is equal to zero. As a result, the measuring rangeincreases not just by the factor of the current distribution betweenconductors 18 and 22, but also by the product of the currentdistribution (ratio of the total current to the portion of the measuredcurrent ratio) and the measuring range enhancement of the current sensor(ratio of the “normal” measuring range to the measuring range in thearrangement according to the present invention). If, for example, in thearrangement according to FIG. 9b, only the half current (currentdistribution factor 2) is conducted via conductor 18, the result is ameasuring range enhancement of 1.36 and an enhancement of the linearmeasuring range to 2.72 times the amount. A conventional current sensorwith 75 amperes could, upon application of the present invention,consequently be brought to the linear measuring range of 190 amperes. Acurrent distribution factor of 5 results in a measuring rangeenhancement of the current sensor to approximately 1000 amperes.

[0043] An even higher current distribution (for example, factor 9)results in a magnetizing of the short-circuit core through theadditional conductor 22, and as a result, a positive, linearity error athigh currents. In order to facilitate even greater measuring rangeshere, the distance of the additional conductor to the short-circuit coremust be increased at a specified distance of (primary) conductor 18. Byvarying the current distribution and/or distances, the current sensormay therefore be set to the maximum measuring range.

[0044] In FIG. 10, the effect of shielding plate 23 in an arrangementaccording to FIG. 9b as opposed to a conventional arrangement accordingto FIG. 9a is shown, the secondary coil having been left out in both.Through shielding plate 23, the magnetic field generated by conductors18 and 22 is further homogenized, and conducted via the section of thecore 19, showing in core 19 only via the gap 20.

1. A Current transformer for a compensating current sensor having aprimary conductor (1), a secondary winding (2), and a soft-magnetic core(4), in which the secondary winding (2) does not completely wind aroundthe core (4), in which the primary conductor (1) and the secondarywinding (2) are electrically insulated from one another and aremagnetically intercoupled via the core (4), and in which the primaryconductor (1) is provided for injecting a current to be detected and thesecondary winding (2) is provided to detect a compensating current,characterized by an additional conductor (3), which is electricallyconnected in parallel to the primary conductor (1) and is arranged inthe area of the core (4), at which a flux rise occurs, with the fluxrise being at least partially compensated for by the flux, which iscaused by the current flowing in the additional conductor (3).
 2. Thecurrent transformer according to claim 1, characterized in that theadditional conductor (3) is arranged beside the core (4) in such a waythat only certain areas of the core (4) are detected by its magneticflow.
 3. The current transformer according to one of the previousclaims, characterized in that at least a second additional conductor(16, 17) is provided, which is electrically connected in parallel to theother two (1, 3) and is arranged beside the core (4) in such a way thatonly certain areas of the core (4) are detected by their magnetic flow.4. The current transformer according to one of the previous claims,characterized in that the primary conductor (1) and the additionalconductor(s) (3, 16, 17) conduct [currents] of different strengths, withthe sum of the currents giving rise to the current to be detected. 5.The current transformer according to one of the previous claims,characterized in that the primary conductor (1) and the additionalconductor(s) (3, 16, 17) exhibit number of turns that are less than one.6. The current transformer according to claim 5, characterized in thatthe primary conductor (1) and the additional conductor(s) (3, 16, 17)are formed by an essentially rectilinear conductor in the area of thecore (4).
 7. The current transformer according to claim 5, characterizedin that the primary conductor (1) and the additional conductor(s) (3,16, 17) are formed by an essentially U-shaped conductor in the area ofthe core (4).
 8. The current transformer according to claim 5,characterized in that the primary conductor (1) and/or the additionalconductor(s) (3, 16, 17) are formed by a one-piece slotted conductor. 9.The current transformer according to one of the previous claims,characterized in that a return conductor (5) is provided, which besidethe core (4), is arranged on the side of the core (4) opposite theadditional conductors (3, 16, 17) and which is traversed in the oppositedirection of the additional conductors (3, 16, 17) by the current to bedetected.
 10. The current transformer according to one of the previousclaims, characterized in that the primary conductor (1), the additionalconductor(s) (3, 16, 17), the secondary winding (2) and the core (4) aresurrounded at least partially by a shielding plate (6).